In the field of electro-optics, systems exist which employ devices for discerning the wavelength of received radiant energy. For example, a source of optical radiant energy, such as earth reflected sunlight may provide a beam that can be used for discerning characteristics of the earth's radiant source. In most receiver systems compatible with such a source, it is desirable to determine the characteristics of the beam such as the spectral content and associated signal strength.
Many optical systems are adapted to perform this detection analysis by utilizing optical filters positioned in the optical path. In this way, the spectral bands of interest may be further analyzed with detection devices such as a photomultiplier tube or solid state detector.
Earth imaging from space by sensors has provided scientists and engineers with new means and insight for determining the magnitude and extent of earth resources. Investigations have proven the value of this data in many fields of interest such as geology, oil and mineral exploration, biomass distribution, forestry, agriculture, and urban and rural development. Researchers involved in the study and application of data in these fields have a need for finer spatial and spectral resolutions and for additional spectral bands.
Analysis of earth imagery from both spaceborne and aircraft instruments has underscored the usefulness of an earth imaging instrument that offers a multiplicity of spectral bands with finer spatial resolution. As many as 12 bands in the Visible-Near (infrared) (VIR/NIR, 0.4-1.0 m), 12 bands in the Short Wave IR (SWIR, 1.0-2.5 m), and a spatial sharpening band are desired.
This desire for analyzing a multiplicity of bands has led to a consideration of various spectrally dispersive instrument designs to provide the desired capability. Prior systems achieved spectral band selection by numerous techniques such as fixed position filters, filters directly deposited on the detectors, a movable filter wheel or tray, and the use of a spectrometer or an interferometric device.
The prior art systems were deficient in several respects. For example, the fixed position filters and filters mounted on the detectors are limited in number of filter/detector combinations that could be accommodated in the instrument focal plane. The movable wheels or trays are large and the use of several of these devices in a focal plane to provide simultaneous readout of several bands is extremely difficult. The use of a spectrometer to provide a continuum of bands is wasteful of the incoming energy because of the inefficiency of the grating or prism spectral dispensing element, and because some of the energy falls within bands of no scientific interest. Furthermore, band selectivity requires an area detector array of large size which is difficult to fabricate and cool, and to obtain uniformity of response further complicates the associated signal processing. An interferometric device, such as the Fabry-Perot interferometer, can provide selection of spectral bands but only one band at a time, and to place several of these devices in a focal plane for simultaneous band selection at different wavelengths is impracticable.